As a guide to a better understanding of certain features of the present invention, it may be noted that at the present time there is a wide variety of corrosion-resistant steels available to the art. The selection of any particular grade by the user largely depends upon the combination of characteristics sought, namely, mechanical properties, corrosion cracking resistance, intercrystalline corrosion resistance, workability etc.
Most of practical advantages of these steels depends on both chemical composition of steel and process for the manufacture thereof, and more particularly such an important step of the process as thermal treatment.
Steels used in welded constructions of large dimensions subjected to considerable stresses and aggressive medium attacks are to meet especially rigid requirements. Such steels must show a high level of strength properties, corrosion cracking resistance and intercrystalline corrosion resistance. Yet, such steels must possess of good plasticity sufficient to lend themselves to a variety of forming and machining technological operations.
There is well known and widely used a corrosion-resistant martensitic steel containing in weight percent: carbon, 0.20; manganese, 1.00; silicon, 1.00; phosphorus, 0.040; sulphur, 0.030; chromium, 15.00 to 17.00; nickel 1.25 to 2.50; and remainder iron. This steel is comparatively inexpensive. After hardening from 1050.degree. C. and temper at 315.degree. C. this steel has a tensile strength of 140 kgf/mm.sup.2 and 0.2 percent yield strength of 98 kgf/mm.sup.2. As a result, this steel is suitable for such parts as spindles, gears, racks etc. However, the steels of the type described show extremely unstable structure. Up to 40 percent of delta-ferrite may be contained in the steel structure according to its chemical composition variations. This leads to deterioration of forgeability and ductility, to considerable decrease of transverse impact strength, and to increase of mechanical anisotropy. Again, stressed parts made of said steel are subjected to intensive corrosion cracking when maintained in aggressive media (for example, hot strong chloride solution).
There is also known a corrosion-resistant austenitic steel containing in weight percent: carbon, 0.08; manganese, 2.00; silicon, 1.00; phosphorus, 0.045; sulphur 0.030; chromium, 17.0 to 19.0; nickel, 9.0 to 12.0; titanium to carbon ratio being at least 5 to 1; remainder iron. Said steel shows rather high ductility and good workability. Forged pieces and bars have in austenitic state (austenitization at 1050.degree. C.) a tensile strength of 53 kgf/mm.sup.2, a 0.2 percent yield strength of 21 kgf/mm.sup.2, an elongation of 40 percent and a reduction in area of 50 percent. This grade of steel is costly, however, because of the rather high alloy content, particularly because of large amount of nickel used therein. Moreover, said steel has a low strength level and an inclination to corrosion cracking, particularly when chlorides accumulation takes place.
There is known an austenitic alloy of high nickel content, which alloy contains in weight percent: carbon, 0.10; manganese, 1.50; silicon 1.00; chromium, 19.0 to 23.0; nickel 30.0 to 35.0; titanium, 0.15 to 0.60; remainder iron. This alloy possesses a high corrosion cracking resistance in strong chloride solutions (42 percent MgCl.sub.2 solution boiling at 154.degree. C., or 0.5 percent NaCl solution boiling at 100.degree. C., etc.). Its mechanical properties are about the same as the corrosion-resistant austenitic steel, being a bit less ductile. Tubes manufactured of said type alloy of high nickel content have in austenitic state a tensile strength of 49 kgf/mm.sup.2, a 0.1 percent yield strength of 21 kgf/mm.sup.2, an elongation of 30 percent. But said type alloy is even more expensive than the austenitic steel mentioned hereinabove because of significantly higher nickel content.
There is also known a corrosion-resistant ferritic steel containing in weight percent: carbon, 0.08; manganese, 1.00; silicon, 1.00; phosphorus, 0.040; sulphur, 0.030; chromium, 11.5 to 14.5; remainder iron. Said steel is of less cost than the corrosion-resistant austenitic steel mentioned above. However, said type steel has a low workability due to elevated overheating sensitivity and inclination to heat embrittlement.
Weldability is one of the important characteristics of steel. The austenitic steels are easily weldable, but they have disadvantages described hereinbefore. The alloys of high nickel content are difficult to weld due to cracks appearing at the near-to-weld zone. Overheating-sensitive ferritic steels fail to provide impact strength of required values due to intensive grain growth.
It is generally required on welding the martensitic steels to preheat the parts to be welded up to temper temperatures ranging from 200.degree. to 300.degree. C. in order to keep them free from cold hardening cracks appearance. This results in considerable complexity and expensiveness of weld process of the martensitic steels.
A group of high-strength corrosion-resistant steels with reversible adjustable transformation on temper of tempered martensite to austenite (.alpha..fwdarw..gamma.) have been recently developed. Said steels enjoy a successful combination of high tensile strength inherent in martensitic steels and good ductility, toughness and workability inherent in austenitic steels. One of these steels (cf. "Transaction ASM" 62, No. 4, 1969, pp. 902-914) contains in weight percent: carbon, 0.10; manganese, 0.40 to 0.90; silicon, 0.20 to 0.80; chromium, 11.5 to 13.5; nickel 5.0 to 6.5; molybdenum, 1.2 to 2.0; remainder iron. Said steel provides for forged pieces having a tensile strength of 85 kgf/mm.sup.2, a 0.2 percent yield strength of 63 kgf/mm.sup.2, an elongation of 15 to 18 percent, a reduction in area of 50 percent and a Charpy test impact energy level of 11 kgf.m.
To develop such properties of the steel the method of manufacture of the steel, and particularly the thermal treatment step thereof contributes greatly.
The process for the manufacture of such a steel resides in preparing a molten mass, pouring the molten mass into a mould and permitting it to solidify therein followed by cooling an ingot produced.
The ingot or forged piece is then subjected to thermal treatment comprising hardening constituting an oil or air cooling, and high temper, which result in developing up to 30 percent of austenite in the structure.
However, the field of usefulness of such a seel is rather limited due to lack of austenite stability at low-temperature heatings. Austenite developed in the structure of articles is destabilizated with prolonged heating at a temperature of 300.degree. to 350.degree. C. and upon cooling the articles to room temperature it is transformated into untempered martensite, which results in decrease of the impact strength and inclination of the steel to corrosion cracking.
Moreover, temper at 590.degree.-600.degree. C. used for said steel to develop a maximum of austenite in the structure fails to fully relieve them from residual stresses after hardening. The residual stresses of a high level constitute a danger of crack formation on cooling ingots, forged pieces and articles of large dimensions fabricated of these steels.